A feline lentivirus resembling human immunodeificency virus (HIV) was siolated in our laboratory from domestic cats with immunodeficiency syndrome and was named feline immunodeiciency virus (FIV). The hallmark of FIV infection in cats is the increased susceptibility of the infected animals to secondary infections much similar to what has been observed for HIV infection of humans. In addition, there are experimental results suggesting a defect in T cell immunity in the infected cats. The use of FIV infection of cats as a small animal model for human AIDS should therefore be considered because it is far more economical, more rapid, and much closer model for human AIDS that those of feline leukemia virus or by other animal lentiviruses with the exception of the primate models. The purpose of this project is to use the FIV model of AIDs to evaluate the role of T lymphocytes in the immunopathogenesis of FIV-FAIDS. In this study, feline reagents and functional assays needed to evaluate feline T lymphocyte network will be developed. These reagents and procedures will then be used to monitor the T-cell functions during various stages of FIV infection in cats. This study will also attempt to address the following points of FIV infection which is still unknown: (1) The identity of the T-cell subset(s) that is/are infected by FIV and the role of T cells as antigen presenting cells. (2) If subsets of T-cells deficiencies do exist then the role of these deficiencies in the immunopathogensis of FIV- FAIDS. The overall significance of this project is to identify the mechanism(s) by which AIDS occurs in animals and to develop immunological approaches that could be used to prevent AIDS virus infection. The results from these studies should help identify prophylaptic and therapeutic strategies that will help combat HIV infection of humans.